1. Field of the Invention
The present invention relates to a liquid crystal composition suitable for use mainly in an active matrix (AM) element, and an AM element comprising the composition.
2. Related Art
A liquid crystal display element is classified, based on an operating mode of a liquid crystal, into phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment and the like. It is classified into passive matrix (PM) and active matrix (AM) based on a driving mode. PM is further classified into static, multiplex and the like, and AM is classified into thin film transistor (TFT), metal insular metal (MIM) and the like. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. A liquid crystal display element is classified, based on a light source, into a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both.
These elements comprise a liquid crystal composition having suitable characteristics. To improve the general characteristics of the composition can lead to the production of an AM element having good general characteristics. The general characteristics of the composition are related with those of an AM element as summarized in Table 1. The general characteristics of the composition shall be explained further based on an AM element on the market. The temperature range of a nematic phase is related with the temperature range in which the element can be used. Desirable upper limit temperature of a nematic phase is 70xc2x0 C. or more and desirable lower limit is xe2x88x9220xc2x0 C. or less. The viscosity of the composition correlates with the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, the composition desirably has low viscosity, and low viscosity at low temperature is more desirable.
Optical anisotropy of the composition correlates with the contrast ratio of the element. To attain a maximum contrast ratio of the element, a product (xcex94nxc2x7d) of the optical anisotropy (xcex94n) of the composition and the cell gap (d) of the element should be designed to be approximately 0.45 xcexcm. Accordingly, the optical anisotropy of the composition is mainly in the range of 0.08 to 0.12. Low threshold voltage of the composition contributes to low electric power consumption and high contrast ratio of the element. Accordingly, low threshold voltage is desirable. High specific resistance of the composition contributes to high voltage holding ratio and high contrast ratio of the element. Accordingly, a liquid crystal composition having high specific resistance in the initial stage is desirable. Further, it is desirable that the composition still has high specific resistance even after a long-term use.
A liquid crystal composition having lower threshold voltage is specifically desired for a transmission type AM element so that it is used at large contrast ratio and low electric power consumption. The conventional compositions are disclosed in the following patent documents; JP-A 10-204016/1998 (U.S. Pat. No. 6,007,740), JP-A 10-204436/1998 (U.S. Pat. No. 5,961,881), JP-A 3051/2001, JP-A 3053/2001, JP-A 123170/2001, JP-A 288470/2001 and WO 01/46336/1989.
The present invention comprises the following items A, B and C.
A. A liquid crystal composition comprising at least one compound selected from the compounds represented by formula (1) as a first component, at least one compound selected from the compounds represented by formulas (2) and (3) as a second component, at least one compound selected from the compounds represented by formulas (4) and (5) as a third component and at least one compound selected from the compounds represented by formulas (6) and (7) as a fourth component: 
xe2x80x83wherein, R1 represents alkyl; Z1 and Z2 each independently represent a single bond or xe2x80x94(CH2)2xe2x80x94; X1 and X2 each independently represent hydrogen or fluorine; and Y1 represents fluorine or xe2x80x94OCF3.
B. A liquid crystal composition comprising at least one compound selected from the compounds represented by formula (2), at least one compound selected from the compounds represented by formula (3), at least one compound selected from the compounds represented by formula (4) and at least one compound selected from the compounds represented by formula (6): 
xe2x80x83wherein, R1 represents alkyl, Z1 and Z2 each independently represents a single bond or xe2x80x94(CH2)2xe2x80x94, X1 represents hydrogen or fluorine, and Y1 represents fluorine or xe2x80x94OCF3.
C. A liquid crystal display element comprising the liquid crystal composition described above.